The present invention relates to a ball screw particularly having a structure capable of circulating balls rolling in a loaded rolling passage formed between a screw shaft and a nut member by a return pipe fitted to the nut member.
One known example of such a ball screw is composed of a screw shaft to which a ball rolling groove is formed and a nut member to which a loaded ball rolling groove corresponding to the ball rolling groove formed to the screw shaft. These ball rolling groove and loaded ball rolling groove constitute, in combination, a loaded ball rolling passage in which balls are rolled. One and other ends of the loaded ball rolling passage are connected by a return pipe fitted to the nut member in a manner such that the ball rolling in the loaded ball rolling passage is scooped at the one end thereof by the return pipe and returned to the loaded ball rolling passage at the other end thereof. The ball screw having the above structure is shown, for example, in Japanese Utility Model Laid-open (KOKAI) Publication No. HEI 6-69502.
FIG. 9 shows one known example of the return pipe 1 having both opened ends, to which a pair of tongue pieces 2a, 2b are formed to be fittable into the loaded ball rolling passage. The balls, which have rolled in the loaded ball rolling passage formed to the periphery of the screw shaft, collide with the tongue pieces 2a, 2b and then are scooped thereby so as to be introduced into the return pipe 1. The balls thereafter circulate through the return pipe 1 and then again return to the loaded ball rolling passage. Notches or cutouts 3 are also formed near the tongue pieces 2a, 2b at both the opened ends of the return pipe 1 for preventing the balls from interfering with the ball rolling groove formed to the screw shaft.
However, in recent years, it has been required to carry out a high feed operation of a machine tool or the like mechanism, and hence, to render the ball screw rotate ABL at a high rotating speed. In a case where the ball screw is used with an increased D/N value (which is a value obtained by multiplying a rotating number per one minute of the screw shaft by a rotating diameter of the ball), there are fears such that stress concentration will be liable to occur on root portions of the tongue pieces 2a, 2b due to repeated collision of the balls, which results in metal fatigue at these root portions, which are hence damaged.